Using an element which has a transistor structure as a memory element which can be used as a ROM (Read Only Memory) has been conventionally proposed.
For example, Patent Document 1 discloses an eFuse (Electronic Fuse) in which two terminals, a cathode and an anode, are provided in a multilayer structure of polysilicon/silicide/silicon nitride film, which has the same wire structure as that used in common logic LSI processes. By allowing a large electric current to flow through this eFuse, the eFuse is heated, and the resistance value between the two terminals varies. Although Patent Document 1 fails to disclose the technique of using the eFuse as a memory cell, Patent Document 1 discloses a configuration where two transistors (select transistors) and an eFuse are connected in series such that a large electric current flows.
Patent Document 2 discloses an electrically-programmable fuse element. Patent Document 2 discloses a configuration where this fuse element and two MOS transistors (select transistors) are connected in series for operations of programming (writing) and reading.
On the other hand, Patent Document 3 of the present applicant proposes a novel memory transistor which is capable of reducing the power consumption as compared with conventional memory transistors. This memory transistor uses a metal oxide semiconductor in the active layer (channel). This memory transistor can irreversibly change to a resistor state which exhibits an ohmic characteristic due to Joule heat produced by the drain current, irrespective of the gate voltage. Using such a memory transistor enables to make a voltage for writing lower than the voltages in Patent Documents 1 and 2, and hence, the power consumption can be reduced.
Patent Document 3 also discloses a memory cell consisting of one memory transistor and one select transistor. Patent Document 3 also discloses forming a memory transistor in an active matrix substrate of a liquid crystal display device, for example.
Note that, in the specification of the present application, the operation of changing a metal oxide semiconductor of this memory transistor to a resistor state is referred to as “writing operation”. In this memory transistor, the metal oxide semiconductor is a resistor after writing, and therefore, the memory transistor does not work as a transistor. However, in this specification, it is referred to as “memory transistor” even after transition to the resistor. Likewise, even after transition to the resistor, terms such as gate electrode, source electrode, drain electrode, channel electrode, etc., which are constituents of a transistor structure are used.